1. Field of the Invention
The present invention relates to a laser beam scanning optical apparatus, and more particularly to a laser beam scanning optical apparatus which is employed in a laser printer, a digital copying machine or the like as printing means.
2. Description of Related Art
Recently, a laser beam scanning optical apparatus which is employed in a laser printer, a digital copying machine or the like as printing means is structured to be capable of printing with a high density for improvement of picture quality. Accordingly, the beam spot on a scanning surface (photosensitive member) must be small, and the focal depth is small. When the environments change, especially when the optical apparatus heats, thereby causing thermal expansion of the optical elements and/or a holder thereof, the image point shifts and comes before or after the scanning surface. The shift of the image point is not allowed in order to keep picture quality.
In order to solve the problem, Japanese Patent Laid Open Publication No. 2-51119 discloses that a detector with a single grating filter detects a shift of the image point and that a lens Is moved to a proper position according to the result of the detection. Japanese Patent Laid Open Publication No. 4-155304 discloses that a detector which is located near the scanning surface and has a knife edge arranged upstream and a photoelectric element arranged downstream respectively along the optical axis detects a shift of the image point and that a lens is moved to a proper position according to the result of the detection.
According to the former prior art (2-51119), because only one grating filter is used, the detector cannot detect whether the shift of the image point is one before the scanning surface or one after the scanning surface. Therefore, the lens must be moved back and forth for focusing, and speedy focusing cannot be carried out. According to this art, it is impossible to carry out focusing during an interval of printing between pages (several hundred milliseconds), and it is likely that the image point shifts during continuous printing of pages.
According to the latter prior art (4-155304), although the detector can detect whether the shift of the image point is one before the scanning surface or one after the scanning surface because of the knife edge, the detection is shown by a single wave and accordingly is unstable. In other words, the performance of the detection is poor.